Well you've come to the right place my friend! I'm sure you'll get a ton of info here, but I'll start off simple and get you started...
1. First things first, you need to update your game. If you downloaded it from steam its already good to go, but if you bought the disc version your going to need the updates. Go to this website http://www.sc4devotion.com/forums/ and register. Once you've registered go back to the home page of sc4devotion and on the left about half way down the page there will be a "EA Support" button. Click it. That will take you to another page. At the bottom of that page there will be a link that says "I accept the terms..blah blah blah," click it. From here there will be a page with several files. The first one you need is the one on the bottom- SC4 Update for rush hour/deluxe. Click it and on the next page it will give you several update options depending on what version(Your location) you have. I'm assuming your North America in which case you would pick sku update 1. Upon clicking the download link it will immediately download the file. You will need to unzip this file once its finished, it will be located in your downloads folder on your computer. Once its been unzipped all you need to do is double click it and follow the directions. It will automatically place the update where it needs to go by itself. Once that update is completed, its time to go to the next one.
Going back to the original page where you selected the SC4 update there will be another link called "Building Architect Tool," click it. The update you need is the second from the top called "Simcity 4 Buildings Update," its the one that is 14.4MB. Do the same thing with this update as you did with the first. Remember all you need to do is unzip it and run it. It will do all the work for you after that.
2. Now we have to make 100% sure your game is up to date. Go to your desktop and locate the shortcut for Simcity 4. Right click it and choose "properties." Under the "Shortcut" tab click "open file location." This will bring you to where the exe application file is located. Right click the one that says "application" under type. From here click "properties" once again and click on the "details" tab. This will tell us if your game is updated. Here the "file version" is listed. If the game is fully updated it will say : 1.1.641.0, If this is not what is listed, then you have to re-run the updates.
3. Now assuming your game is updated I'll briefly go over the downloading process. Every time you locate a file on this site or another make sure you read the description! Normally there will be a list of "dependencies" that are needed for the file to work in the game. Pay attention to this because they are important. Many downloads will have links to the dependencies, by clicking them they will bring you to the appropriate page to download the dependency. If the links to the dependency are broken or there isnt one at all you can usually do a google search of the name of the dependency and find the file that way. If you still can't locate it, then its probably time to move on to another download. Also make sure you read the readme's of every file you download. Sometimes a download will not list the dependency in the description and its important to read the readme to make sure you don't need any dependencies to make the file work. If you find a download that doesn't list any dependencies in the description and doesn't say anything in the readme(or doesn't have a readme) your good to go and don't need any more files to make it work. Moving on, after you've downloaded the file you want in the game and any dependencies you may need, you need to unzip the download. After the file is unzipped it will either have a installer in which case you just run it or it will have a number of files. All of these files need to be put in your "Plugin" folder. To get to this folder you first need to go to "my documents" Here there should be a folder named "Simcity 4." In that folder there will be a subfolder named "Plugins" This is where every mod or file you download for the game needs to go. I would suggest organizing all of your downloads. In mine I have a folder for dependencies another for buildings and one for mods. I also make a folder for every download with the appropriate name so that if I never need to find a file, its much easier.
I'll stop there for now. Let me know when you have your game updated and I'll help you out more with the downloading process. Cheers!